This invention relates to apparatus for converting a continuous laser beam into one or more pulsed laser beams in which the duty cycle can be set at any one of an infinite number of values between a minimum and a maximum value. More particularly, this invention relates to a system for modifying a continuous laser beam and directing it in the form of one or more pulsed beams, in which the duty cycle can be varied, at a moving paper web for the purpose of perforating rows of holes in the web.
In the manufacture of cigarettes, for example, it is a common practice to perforate the cigarette paper or tipping paper in rows, for the purpose of providing smoke dilution. Before the laser became available, the perforations were made by mechanical means or with electrical spark perforators. With the advent of the laser, it became possible to achieve greater precision and speed than had previously been possible.
In creating a row of perforations using a laser, it is usually the practice to move the paper web while pulsing the laser beam. The maximum speed of the process is thus governed by the frequency at which the beam can be pulsed and the speed at which the web can be moved. With currently available laser equipment, the internal pulsation frequency of the laser is a limiting factor. Because of this, other means, external to the laser, for interrupting a continuous beam have been sought.
In addition, it is frequently desired to perforate the web at more than one location at a time. For example, it may be preferable to have two or more parallel rows of perforations. However, because of the increased capital and operating expenses and space limitations, it is not desirable to use more than one laser to achieve such perforations.
One type of system that has been developed to meet these needs uses mechanical rotary shutters or "beam choppers," to periodically interrupt a beam at a desired pulsation frequency. Portions of the shutters may be made reflective in order to direct the pulsed beam to several different targets sequentially, utilizing some of the energy that would otherwise be wasted in the "off" portion of the cycle. An example of this type of system is shown in U.S. Pat. No. 4,118,619, which discloses a plurality of coaxial discs provided with a predetermined pattern of apertures, reflective portions, and opaque portions.
Such systems do not permit of much adjustment once assembled. The pulsation characterisics of the output beam can be changed only to the extent that they are affected by changes in the rotational speed of the disc. The higher the rotational speed becomes, the greater becomes the pulsation frequency and the shorter becomes the pulse duration, or "on" time. It is not possible to adjust the pulsation frequency without at the same time changing the pulse duration. In any case, the duty cycle, or ratio of "on" time of the beam to the total of "on" plus "off" time, would remain constant. To adjust one of these characteristics without affecting the others in the system of U.S. Pat. No. 4,118,619, for example, would require disassembling the system and re-assembling it with a new set of discs.
The capacity to make such adjustments more easily is desirable because equipment with such capabilities could be used for producing more than one type of product, without the need for reassembly of the equipment during the change-over between products. For example, it may be necessary to produce holes of different sizes for different types of cigarettes. For others, it may be necessary to place the perforations closer together or farther apart.